One hydraulic circuit in the related art for a construction machine as illustrated in FIG. 1 includes first and second variable displacement hydraulic pumps 2 and 3 and third and fourth fixed displacement hydraulic pumps 4 and 15 connected to an engine 1; a first control valve 5 installed in a flow path of the first variable displacement hydraulic pump 2 and shifted to control hydraulic fluid supplied to hydraulic actuators that drive a boom, a bucket, and a traveling device in response to pilot signal pressure supplied from the fourth hydraulic pump 15; a second control valve 5a installed in a flow path of the second variable displacement hydraulic pump 3 and shifted to control hydraulic fluid supplied to hydraulic actuators that drive a swing device, an arm, and the traveling device in response to the pilot signal pressure supplied from the fourth hydraulic pump 15; a hydraulic motor 9 connected to the third fixed displacement hydraulic pump 4; a cooling fan 10 connected to the hydraulic motor 9 and rotated to discharge cooling wind to an oil cooler 11 to lower temperature of the hydraulic fluid that is drained to a hydraulic tank T through a return flow path 16; a temperature sensor 13 detecting the temperature of the hydraulic fluid in the hydraulic tank T; an electric relief valve 12 installed in a discharge flow path 17 of the third hydraulic pump 4 to control hydraulic pressure that drives the hydraulic motor 9 so as to variably control a rotating speed of the cooling fan 10; and a controller 14 controlling the hydraulic pressure that drives the hydraulic motor by varying the set pressure of the hydraulic motor 9 by varying set pressure of the electric relief valve 12 according to a detection signal from the temperature sensor 13.
Here, the detailed description and illustration of spools of the first and second control valves 5 and 5a, which are shifted to control the hydraulic fluid supplied from the first and second hydraulic pumps 2 and 3 to the hydraulic actuators in response to pilot signal pressure that is supplied from the fourth hydraulic pump 15 through shifting of a pilot pressure generation device 6, are omitted.
In the drawing, the reference numeral “8” denotes a relief valve installed in a pilot flow path 18 of the fourth hydraulic pump 15 to drain the hydraulic fluid to the hydraulic tank T when a load that exceeds pressure set in the fourth hydraulic pump 15 occurs.
Accordingly, by shifting the spools of the first and second control valves 5 and 5a through the shifting of the pilot pressure generation device 6, a working device such as a boom is driven by the hydraulic fluid that is supplied from the first hydraulic pump 2 to the hydraulic actuator, and the swing device is driven by the hydraulic fluid that is supplied from the second hydraulic pump 3 to the hydraulic actuator.
The hydraulic motor 9 is driven by the hydraulic fluid that is supplied from the third hydraulic pump 4 to the discharge flow path 17, and the cooling fan 10 is rotated by the driving of the hydraulic motor 9 to lower the temperature of the hydraulic fluid that returns to the hydraulic tank T through the oil cooler 11 installed in the return flow path 16.
The wind speed of the cooling wind that is discharged from the cooling fan 10 to the oil cooler 11 is in proportion to the rotating speed of the cooling fan 10, and if the rotating speed of the cooling fan 10 is increased, the load pressure of the hydraulic motor 9 is also increased.
In this case, the load pressure of the hydraulic motor 9 is controlled by the electric relief valve 12. That is, if the load pressure of the hydraulic fluid that is supplied from the third hydraulic pump 4 to the hydraulic motor 9 exceeds the set pressure of the electric relief valve 12, the hydraulic fluid having the excessive pressure is drained to the hydraulic tank T through the electric relief valve 12. Accordingly, the rotating speed of the cooling fan 10 can be controlled by the set pressure of the electric relief valve 12.
In the case of driving the working device such as the boom, the temperature of the hydraulic fluid, which returns from the hydraulic actuator having an increased temperature to the hydraulic tank T, is lowered by the cooling wind that is discharged through the cooling fan 10 while the hydraulic fluid passes through the oil cooler 11 installed in the return flow path 16.
That is, a detection signal, which corresponds to the temperature value of the hydraulic fluid in the hydraulic tank T that is detected by the temperature sensor 13, is input to the controller 14, and the controller 14 varies the set pressure by transmitting the control signal to the electric relief valve 12 so as to keep the set temperature of the hydraulic fluid.
For example, if the temperature of the hydraulic fluid in the hydraulic tank T exceeds the set temperature, the set pressure of the electric relief valve 12 is increased to heighten the hydraulic pressure that drives the hydraulic motor 9. Accordingly, the rotating speed of the cooling fan 10 is increased to increase the cooling capacity of the oil cooler 11.
In the hydraulic circuit in the related art for a construction machine illustrated in FIG. 1, the fourth fixed displacement hydraulic pump 15 (that is, the pilot pump) fixedly discharges a constant flow rate in accordance with the rotation of the engine 1. The hydraulic fluid that is discharged from the fourth hydraulic pump 15 is instantaneously used as the pilot signal pressure that shifts the spools of the first and second control valves 5 and 5a when the pilot pressure generation device 6 is shifted.
On the other hand, if the load that exceeds the set pressure occurs in the pilot flow path 18, the hydraulic fluid that is discharged from the fourth hydraulic pump 15 is drained to the hydraulic tank T through the relief valve 8, and this causes a power loss to occur.
That is, the lower loss is as follows.Power loss=(set pressure of the relief valve 8)×(discharge flow rate that is drained to the hydraulic tank T)
Further, since the fourth hydraulic pump 15 is separately connected to the engine 1, the structure of the hydraulic circuit becomes complicated to cause the increase of the production cost.
Another hydraulic circuit in the related art for a construction machine as illustrated in FIG. 2 includes first and second variable displacement hydraulic pumps 2 and 3 and a third fixed displacement hydraulic pump 4 connected to an engine 1; a first control valve 5 installed in a flow path of the first variable displacement hydraulic pump 2 and shifted to control hydraulic fluid supplied to hydraulic actuators that drive a boom, a bucket, and a traveling device in response to pilot signal pressure supplied from the third hydraulic pump 4; a second control valve 5a installed in a flow path of the second variable displacement hydraulic pump 3 and shifted to control hydraulic fluid supplied to hydraulic actuators that drive a swing device, an arm, and the traveling device in response to the pilot signal pressure supplied from the third hydraulic pump 4; a hydraulic motor 9 connected to the third fixed displacement hydraulic pump 4; a cooling fan 10 connected to the hydraulic motor 9 and rotated to discharge cooling wind to an oil cooler 11 installed in a return flow path 16 of the first and second hydraulic pumps 2 and 3 to cool the hydraulic fluid that returns to a hydraulic tank T; a temperature sensor 13 detecting the temperature of the hydraulic fluid in the hydraulic tank T; an electric relief valve 12 installed in a discharge flow path 17 of the third hydraulic pump 4 to control hydraulic pressure that drives the hydraulic motor 9 so as to variably control a rotating speed of the cooling fan 10; a controller 14 controlling the hydraulic pressure that drives the hydraulic motor by varying the set pressure of the hydraulic motor 9 by varying set pressure of the electric relief valve 12 according to a detection signal from the temperature sensor 13; a pilot pressure generation device 6 installed in a pilot flow path 18 connected as a branch to a flow path of the third hydraulic pump 4 and shifted to supply pilot signal pressure to the first and second control valves 5 and 5a; a pressure reducing valve 7 installed in the pilot flow path 18 to supply the hydraulic fluid from the third hydraulic pump 4 to the pilot pressure generation device 6 by a set pressure of a valve spring 7b, and shifted to drain the hydraulic fluid to the hydraulic tank T if a load that exceeds the set pressure of the valve spring 7b occurs in the pilot pressure generation device 6; and a relief valve 8 installed in the pilot flow path 18 between the pressure reducing valve 7 and the pilot pressure generation device 6.
Since the pilot flow path 18 is connected as a branch to the discharge flow path 17 of the third hydraulic pump 4 for the cooling fan 10 and the pressure reduction valve 7 is installed in the pilot flow path 18, a separate fixed displacement hydraulic pump is not used, and thus a power loss can be minimized.
On the other hand, in the case of operating the pilot pressure generation device 6 that uses the hydraulic fluid from the third hydraulic pump 4 for the cooling fan 10 (see a curve “a” in FIG. 3), the flow rate of the hydraulic fluid of the third hydraulic pump 4 that is supplied to the hydraulic motor 9 is instantaneously reduced. Due to this, the revolution of the cooling fan 10 is abruptly reduced (for example, 1109 RPM→407.5 RPM) (see a curve “b” in FIG. 3), and thus the cooling effect is lowered.
Further, since the revolution of the cooling fan 10 is repeatedly changed between high RPM and low RPM depending on the operation of the pilot pressure generation device 6, noise (mechanical sound generated due to the irregular revolution of the cooling fan 10) occurs. Due to the irregular noise that occurs due to the change of the revolution of the cooling fan 10, an operator is unable to perform the operation smoothly.